Electric alignment of fibers for the manufacture of composite materials

ABSTRACT

An apparatus and method for the production of composite material parts by aligning reinforcing fibers through application of electric fields. The fibers are in the form of rods preimpregnated with a matrix material or coated with a sizing, and aligned within a dielectric alignment fluid. By providing appropriate electrodes, an electric field is created which mimics the stress lines exhibited by the final composite part when under stress. The reinforcing fibers align to the electric field, thereby aligning to the stress lines.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to the manufacture of composite materialparts, such as those made of fiber reinforced plastic, metal, orceramic. More specifically, the present invention pertains to anapparatus and method of aligning reinforcing fibers in a dielectricalignment fluid by applying an electric field to orient the fibers asdesired.

2. Description of the Prior Art

The use of fibers as reinforcement in composite materials is well knownin the art. It is also known that optimal reinforcement occurs when thefibers are aligned along critical stress directions in the final part,thus taking full advantage of the fibers' stiffness and strength inreinforcing the part.

Prior art methods of composite part manufacture include manual andautomated production techniques, both of which exhibit undesirablelimitations. Stated generally, manual techniques are very expensive, andautomated techniques are incapable of producing fiber aligned parts inother than a few very specific shapes.

Manual alignment of fibers or continuous filaments involves placing thefibers or filaments in orientations according to the desired strengthcharacteristics of the final part. The fibers or filaments can be placedbetween layers of the matrix material, or can be in the form of a tapecontaining the reinforcing fibers as well as the matrix material.

While this method allows reinforcement of the matrix material invirtually any desired pattern, the cost of production is high. Manualplacement of fibers is labor intensive, and cannot be easily automated.Thus the cost of the final part is higher than it would be if anautomated method was available.

Some solutions to the excessive cost of the manual alignment methodinclude automated techniques such as filament winding, pultrusion,compression and injection molding. Filament winding is a process wherefibers are oriented by wrapping around a mandrel, but this technique isgenerally only useful for producing cylindrical parts. Pultrusioninvolves pulling continuous fibers through a resin bath and then througha heated die to form its cross sectional shape, but this technique canonly produce constant cross section beams and cannot be used to makeparts of arbitrary shape. Compression and injection molding can be usedwith fiber reinforced plastics to produce virtually any shape. Noattempt is made to align the fibers into an advantageous orientation incompression molding, and only limited alignment using flow fields ispossible in injection molding.

Another solution to the excessive cost of the manual alignment method isa method of aligning fibers by applying a magnetic or electric field andallowing fibers to fall through this field, thereby aligning the fibersalong the field lines while they fall to be collected into a mat belowthe electrodes. Magnetic field alignment methods are of limited utility,since the fibers need to be ferromagnetic in order to be influenced intoalignment by a magnetic field. Since none of the advance fibers in usetoday are ferromagnetic, they would need to be coated with aferromagnetic material, substantially increasing the cost of production.

Electric fields are more useful, since they only require that the fiberbe electrically conductive, which many of today's advanced fibers are.Thus no additional processing is required prior to alignment of thefibers in an electric field. There are, however, many problemsassociated with electric field alignment methods. First, the fiberscannot be economically aligned while suspended within the matrixmaterial itself. The matrix material can be in a soft state, allowingfor movement of the conductive fibers, but the high viscosity of thetypical molten matrix makes alignment slow and difficult. The intensityof the electric field would need to be substantially increased so as toincrease the aligning force exerted on the fibers and reduce alignmenttime, but this can only be done to the point at which the matrixmaterial breaks down and the electric field is shorted out.

In addition, fiber to fiber interactions can prevent proper alignmenteither within the matrix or within an alignment fluid. As fibers collidewith each other, they may physically prevent each other from aligning.Also, prior art electric field alignment techniques are generallydirected toward alignment of relatively non-conducting fibers such aswood particles. However, since many of the modern fibers used inproduction of composite parts are highly conductive, the existingelectric field alignment techniques have severe limitations. As theconductive fibers are being aligned, they come into physical contactwith each other, forming a chain which locally shorts out the electricfield in the vicinity of the chain. They may even gather to form longenough chains to short out the electric field entirely. Reducing theconcentration of conductive fibers reduces this effect, but eitherincreases the processing time or reduces the fiber concentration in thevinyl part.

While electric field alignment methods do lend themselves to automatedproduction, they have not provided the flexibility to align fibers intoany desired alignment pattern. Such prior art methods are performedbetween a pair (or multiple pairs) of plate electrodes, and areparticularly well suited for aligning fibers into a single linearorientation. The mechanics of these methods do not easily providefacility for either more complex orientations, or for changing alignmentpatterns while the mat is gathered at the bottom.

Electric field alignment of conductive fibers also suffers from theproblem of field distortion near the bottom of the electrodes. As theconductive fibers collect in a mat at the bottom, they form a conductivesheet which tends to short out the electrodes and thus the electricfield. One attempted solution to this problem is to provide electrodesthat do not reach the bottom, and thus do not contact the fiber mat. Asthe fibers fall through the electric field formed by parallel plateelectrodes, they align according to the resulting parallel electricfield lines. However, near the bottom of the electrodes, the field linesare distorted due to the conductive bottom, and as the fibers reach thebottom of the electrodes and are gathered into the mat, they re-alignaccording to the distortions rather than remaining aligned with thedesired parallel field lines.

In addition, the viscosity of the alignment fluid affects the processingrates and the overall efficacy of the alignment process, however, priorart electric field alignment techniques do not teach whether there isany utility to be gained from proper selection of alignment fluidviscosity.

Another potential solution to the distortions at the bottom of theelectrodes is described in U.S. Pat. No. 4,113,812 to Talbott et al.,where a voltage gradient is applied to the mat horizontally. The problemwith this method is that it is unsuitable for use with compositematerial fibers due to their high conductivity. A mat of highlyconductive fibers would short out the intended voltage gradient andwould generate considerable heat, perhaps sufficient to cause thealignment fluid to boil.

Other limitations in the prior art methods become apparent when athree-dimensional fiber orientation is desired. The technique ofallowing fibers to drop through an electric field and collecting them atthe bottom is generally incapable of producing a non-planar alignment.While an electric field which is not parallel to the collecting surfacecould conceivably be created, the fibers would be subjected to aphysical force as they contact the collecting surface, which would tendto realign the fibers so as to be parallel to the collecting surface.

Another problem with prior art methods of producing composite materialparts is that strong reinforcing fibers tend to be expensive, andprevious automated alignment methods do not make efficient use of thesefibers. The prior art does not disclose an automated method by which thefibers can be advantageously placed in locations of the part where mostneeded. Rather, the entire part must contain the same concentration offibers as is needed in the most critical location of the part.

It is evident that there is a continuing need in the prior art for anefficient and automatable method of aligning fibers for thereinforcement of a matrix material in the manufacture of compositeparts. There are also continuing needs for a method of aligning highlyconductive reinforcing fibers, a method which makes efficient use ofexpensive reinforcing fibers, a method capable of producing athree-dimensional alignment pattern, and an apparatus capable ofeffectuating such methods.

SUMMARY OF THE INVENTION

In accordance with the present invention, reinforcing fibers are alignedunder the influence of an electric field. The fibers are pre-impregnatedwith matrix material to form prepreg rods with the fibers oriented alongthe length of the prepreg rods, and are dispersed onto the surface of atank filled with a dielectric fluid. The fibers, the matrix material, aconductive coating on the rod or fibers, or a filament within theprepreg rod is conductive, and thus allows the orientation of theprepreg rods to be influenced by application of an electric field.

In accordance with another aspect of the present invention, reinforcingfibers are sized (coated) with a thermal formable material and alignedunder the influence of an electric field. The sized fibers rods arealigned into a preform mat in the shape of the final part, and thenimpregnated with a special low viscosity matrix material.

In one embodiment of the present invention, a tank contains a dielectricfluid which has a density less than that of the prepreg rods. Anelectric field is applied within the dielectric fluid, such that theflow lines of the field correspond to the desired orientation of thefibers. The applied field is of decreasing vertical intensity,terminating at the bottom of the tank with zero intensity. This isaccomplished through the use of resistive or voltage gradientelectrodes, fed from the top with high voltage and shorted at the bottomby the bottom of the tank which is conductive. This arrangement ensuresthat there will be no distortions of the field lines in the vicinity ofthe conductive collecting surface. The prepreg rods are then dispersedonto the surface of the dielectric fluid, and allowed to sink throughthe fluid while under influence of the electric field, and are collectedat the bottom of the tank in their aligned orientation.

In another embodiment of the present invention, the fibers are alignedaccording to the stress lines in the composite material part when inuse. This is done by providing a tank in the shape of the desired part,with electrodes located at load bearing surfaces according to themechanical design of the part. An electric potential is applied betweenthe electrodes, creating electric field lines analogous to the stresslines of the part. As the fibers fall through the dielectric fluid, theyare aligned along these stress lines, optimally reinforcing the partaccording to the physical stress it will endure in operation.

In another embodiment of the present invention, a method is provided foraligning fibers into a three-dimensional orientation. This is done byselecting the dielectric fluid to have the same density as the prepregrods. The rods will not sink toward the bottom, but rather will remainsuspended in the fluid. An electric field is applied, with field linescorresponding to the desired orientation of the fibers. The appliedfield need not be co-planar with the bottom of the tank, since theprepreg rods will not collect at the bottom. If a non-planar electricfield is applied, the resulting fiber orientation will be non-planar.Once alignment is complete, the prepreg rods are bound into theiraligned orientation, and the electric field and the dielectric fluid maybe removed.

In another embodiment of the present invention, a method is provided forpreferentially locating reinforcing fibers in locations of a compositematerial part where they are most needed. Again, a tank containing adielectric fluid is used, and an electric field aligns the fibers. Theprepreg rods added to the tank comprise a mixture of strong prepreg rodsand weaker rods. The strong rods contain strong reinforcing fibers, andthe weaker rods contain weaker fibers, or perhaps no fibers. The strongrods are conductive so as to be influenced by the electric field, whilethe weaker rods are non-conductive or substantially less conductive, soas to be less influenced or uninfluenced by the electric field. Thestrong prepreg rods can be made conductive by use of a conductive fiber,a conductive filament included in the prepreg rod, by use of aconductive coating, or any other method generally known to one skilledin the art. When the electric field is applied, the strong prepreg rodswill align according to the field lines, and will migrate towardlocations of higher field intensity, displacing the weaker rods towardlocations of lower field intensity. This results in a higherconcentration of strong rods in the preferred locations, resulting inreduced material cost since fewer strong rods need to be used.

In another embodiment of the present invention, an apparatus is providedwhich is capable of effectuating the methods of the present invention.The apparatus includes a tank, a dielectric alignment fluid, a source ofhigh voltage and electrodes. The electrodes may be voltage gradient, andcan be in any shape including plates, rods, and spheres. This allows forcreation of virtually any pattern of field lines desired, and forelectronic control of the pattern of field lines by switching electrodeson and off, and controlling the relative voltage and Alternating Current(AC) phase between electrodes during formation of the part.

Accordingly, it is an object of the present invention to provide anefficient method of manufacturing composite materials, wherein thereinforcing fibers are aligned in a dielectric fluid by application ofan electric field.

It is another object of the present invention to provide a method ofaligning fibers according to the stress lines in the composite materialpart when in use.

It is another object of the present invention to provide a method foraligning fibers into a three-dimensional orientation.

It is another object of the present invention to provide a method forpreferentially locating reinforcing fibers in locations of a compositematerial part where they are most needed.

It is another object of the present invention to provide an apparatuscapable of effectuating the methods of the present invention.

These and other objects of the present invention will become apparentwith reference to the drawings, the detailed description of thepreferred embodiment, and the appended claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical apparatus for using electric fields to alignfibers in a dielectric alignment fluid.

FIG. 2 shows prior art field distortion due to the conductive bottom ofthe tank below the electrodes.

FIG. 3 shows undistorted field lines due to resistive or voltagegradient electrodes.

FIGS. 4a and 4b show the construction of one embodiment of a voltagegradient electrode.

FIGS. 5a and 5b are top views of an alignment tank showing field linesdue to rod and plate electrode combinations.

FIG. 6a shows a typical voltage gradient on the face of a voltagegradient plate electrode.

FIG. 6b show distortions in the voltage gradient of FIG. 6a due to ashort on the face of the electrode.

FIG. 7 shows the voltage gradient of a plate electrode simulated usingdiscrete voltage gradient rod electrodes.

FIGS. 8a, 8b, 8c and 8d show various possible grid arrangements of rodelectrodes.

FIG. 9 shows a side view of a prepreg rod, depicting its aspect ratio.

FIGS. 10a, 10b, 10c, and 10d show various possible cross sections ofprepreg rods, depicting the cross section aspect ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed toward a method of aligningreinforcing fibers using an applied electric field, using fibers such asgraphite, carbon, glass, aramide sold under the trademark KEVLAR, boron,silicon carbide, and various metals. Those skilled in the art willrecognize that there are numerous variations possible within the scopeof the present invention. There are many factors which will determinethe exact method and apparatus needed for a particular application,including the material the part is to be made of, the type ofreinforcing needed, the pattern of reinforcing needed, and others. Theseand other considerations will be discussed in describing the preferredembodiments of the present invention.

Some of the details concerning electric alignment of reinforcing fibersaccording to the present invention are contained in an article entitled"USING ELECTRIC FIELDS TO CONTROL FIBER ORIENTATION DURING MANUFACTUREOF COMPOSITE MATERIALS" which was authored by the present inventor andwas published in the Society For the Advancement of Material and ProcessEngineering (SAMPLE) International Symposium, Volume 34, pages 385,1989, and which is incorporated herein by reference.

Referring now to FIG. 1, a representation of a typical alignmentapparatus 10 is shown. The alignment apparatus 10 includes an alignmenttank 12 of a nonconductive material with two plate electrodes 14a and14b, which are shown substantially parallel and facing each other, butwhich could be in any orientation depending upon the alignment patterndesired. A source of high voltage 16 is connected through wires 18a and18b to the electrodes 14a and 14b. Within the tank is an alignment fluid20, which is preferably nonconductive and dielectric. The tank alsocontains a shower head 21, a distribution plate 22, and a flowstraightener 23. A sufficient quantity of prepreg rods 24 is placed onthe distribution plate 22, which is located approximately two inchesbelow the surface of the alignment fluid 20. The pump 25 circulatesalignment fluid 20 from the bottom of the tank 12 to the shower head 21,filtering any accumulated contaminants from the fluid 20. The fluid flowfrom the shower head 21 agitates the prepreg rods 24, causing them tofall through perforations in the distribution plate 22, providing auniform distribution of prepreg rods across the corss section of thetank. The flow straightener 23 consists of vertical baffles which serveto eliminate any turbulence in the downward flow of alignment fluid 20and prepreg rods 24.

A prepreg rod 24 is a rod of matrix material containing reinforcingfibers aligned longitudinally. The alignment fluid 20 is chosen to havea density less than that of the prepreg rods 24, so that when theprepreg rods 24 are placed on the surface of the alignment fluid 20,they sink toward the bottom of the tank 12. Details of the prepreg rodswill be discussed in reference to FIGS. 9, 10a-d.

As the prepreg rods enter the alignment fluid 20, they are subjected toan electric field (not shown) induced by the voltage applied to theelectrodes 14a and 14b. The prepreg rods 24 are conductive, and thisconductive nature causes them to become electric dipoles which thenalign with the electric field. Thus the reinforcing fibers containedwithin the prepreg rods 24 are aligned according to the electric fieldinduced within the tank 12. The prepreg rods 24 fall through thealignment fluid 20 to the conductive bottom 27 of the tank 12, wherethey are collected into a mat of aligned rods 26.

Since the bottom 27 is conductive, there is effectively no electricfield acting upon the prepreg rods once they reach the bottom 27, thusthe electric field orientation can be changed without disturbing theprepreg rods 24 which are already aligned and at the bottom of the tank.In addition, friction prevents the prepreg rods 24 within the mat ofaligned rods 26 from changing their alignment. Thus, the electric fieldcan be changed so as to cause a different alignment of the prepreg rodsin different layers of the mat 26 gathering at the bottom of the tank12. One skilled in the art will recognize that the electric field may bechanged by any number of methods, including changing the voltage appliedto the electrodes, switching to other electrodes, using different shapesof electrodes, and applying various Alternating Current (AC) voltagesources with various phase relationships between the electrodes.

Once the prepreg rods 24 are collected into the mat 26, the alignmentfluid 20 is removed, and the mat can be placed into a mold formanufacture of the desired composite part according to traditionalmethods. Alternatively, the mat 26 may simply be heated in order to bondthe loosely packed prepreg rods 24 together, retaining the fiberalignment and allowing the mat to be stored for later molding.

The alignment fluid 20 must be chosen to optimize the productionprocess. The optimum viscosity of the fluid depends upon the size,shape, and conductivity of the prepreg rods 24, and the magnitude of theelectric field introduced between the electrodes 14a and 14b.Electrohydodynamic theory could predict the optimum combination of theseparameters, however, a workable combination can be much more easilyderived through minimal experimentation by one skilled in the art. Ifthe chosen alignment fluid 20 has a viscosity that is too low, theprepreg rods 24 will oscillate in a rotary fashion about their minoraxis as they descend toward the mat 26, causing a substantial amount ofmisalignment. If the viscosity is too high, then the prepreg rods 24will take too long to fall and too long to align, lengthening theproduction time and increasing the cost. Ideally, the prepregrod-electric field-alignment fluid system will have a damping ratio of0.7 (70% of critical damping). Factors affecting this include the sizeand shape of the prepreg rods, the viscosity of the alignment fluid, theconductivity of the prepreg rods, and the intensity of the electricfield.

The alignment fluid 20 must be chosen to have an adequate dielectricstrength to reduce the possibility of arcing. Since some degree ofarcing is likely to occur anyway, the chosen fluid ideally would benonflammable. The fluid also must not dissolve or otherwise chemicallyreact with the fibers or the matrix material.

In addition, the density of the alignment fluid 20 must be chosen. Ifthe fluid density of the chosen alignment fluid is too low, the rodswill sink too fast causing turbulence and misaligned rods. In addition,the rods may not have sufficient time to fully align. If the fluiddensity of the chosen alignment fluid is too high, then the rods willdescend through the fluid slower than is necessary, resulting in theprocess taking too long. The fluid chosen thus depends upon the specificgravity of the prepreg rods 24 used. For specific gravities of 1.4 to1.6, halogenated hydrocarbon solvents such as FREON refrigerant gas and1,1,1 trichloroethane can be used. For specific gravities of 0.9 to 1.2,light weight mineral oils and low viscosity silicone oils can be used.If necessary, a mixture of two or more fluids would allow for precisecontrol of the density over a wide range of values. In addition, therate at which the prepreg rods 24 descend in the tank can be adjusted bycontrolling the fluid flow rate through the shower head 21.

Referring now to FIG. 2, field lines 40 between parallel plateelectrodes 46a and 46b are shown, including distorted field lines 42caused by the presence of a conducting bottom 44 of the tank. Theconducting bottom 44 is due to either the tank bottom material beingconductive, or the mat of conductive fibers being collected at thebottom. Fibers placed in the tank 48 are correctly aligned by thesubstantially parallel field lines 40 while they are near the top of thetank. As they descend toward the bottom, they are no longer influencedby field lines which are substantially parallel, but rather areinfluenced by the distorted field lines 42 near the bottom 44. This willcause the fibers on the bottom near the plate electrodes 46a and 46b tostand on end rather than remain aligned in the intended planarorientation.

An improvement according to one aspect of the present invention is touse voltage gradient electrodes, such as using plates of a resistivematerial. The resistive plates will act like voltage dividers, graduallylowering the field strength from full strength at the top to zero at thebottom. The resulting field lines are shown in FIG. 3, where the fieldlines near the top of the tank 50 are drawn close together indicating ahigh field strength, while the field lines near the bottom of the tank52 are drawn further apart indicating reduced field strength. Parallelplate electrodes 54a and 54b are of a resistive material, and are bothconnected to the bottom 56 of the tank, which is conductive so as toassure zero field intensity at the bottom. The resulting field lines areparallel and without the distortions shown in the prior art of FIG. 2.

An alternative embodiment of voltage gradient electrodes is shown inFIGS. 4a and 4b, with FIG. 4b showing a side view of two parallel platevoltage gradient electrodes. Each electrode 60 comprises a plurality ofconductive strips 62a-e arranged horizontally and spaced so as to nottouch each other. Resistors 64a-d connect to the copper strips 62a-e asshown, so as to form a voltage divider. Resistor 64e connects the uppermost conductive strip 62a to a conductive plate 66. The size of theplate 66 relative to the overall size of the electrode 60 determineswhich portion of the electrode 60 is voltage gradient. Resistor 64fconnects the lower most conductive strip 62e to a metal collectingsurface 68. The electrode 60 may be insulated with a plastic film 70 orother insulating material to prevent conductive fibers from contactingthe electrode and shorting out the electric field, if necessary. As seenin FIG. 4b, the second electrode 72 is identically connected withresistors and to collecting plate 68. When a source of high voltage (notshown) is connected to the top sections of electrodes 60 and 72, theresult is a full strength electric field near the top, with a verticallydecreasing intensity terminating in zero intensity at the collectingsurface 68.

In a preferred embodiment of the present invention, the resistors are1MΩ, 1/2 watt resistors, with ten 1" copper strips forming the voltagegradient portion of each electrode. The collecting surface 68 ispreferably a metal screen so as to provide a conducting surface forcollection of fibers, while allowing for draining or circulation ofalignment fluid through the bottom of the tank.

One skilled in the art will recognize that many variations of a voltagegradient electrodes are possible within the scope of the presentinvention, including the use of various conducting and insulatingmaterials, various numbers of segments, and various resistive values.

While parallel plate electrodes are ideal for uniaxial alignmentpatterns, they are unsuitable for most other desired alignment patterns.Many composite parts have stress lines that are not uniaxial, and thus auniaxial alignment of reinforcing fibers is not optimal. Since thedesired orientation of reinforcing fibers is along stress lines in thefinal part, and since fibers will align according to an applied electricfield, it is advantageous to apply an electric field that imitates thestress lines the final part is expected to exhibit. In order to providean electric field that is not uniaxial, electrodes other than plates arerequired. In general, placing electrodes at positions corresponding toload bearing surfaces will result in an electric field that imitates thestress lines exhibited in the final part.

Referring now to FIGS. 5a and 5b, two examples of alignment patternsusing plate electrodes and a single rod electrode are shown. FIG. 5ashows a top view of an alignment tank bordered with plate electrodes80a-d which are connected together and grounded. Rod electrode 82 isconnected to a high voltage source (not shown), and is a voltagegradient electrode, constructed of a continuously resisted material, ina manner similar to the voltage gradient electrodes of FIGS. 4a and 4b,or may simply be a string of resistors soldered together end to end withan insulating cover. The plate electrodes 80a-d and the rod electrode 82are connected to a conductive collecting surface at the bottom, in afashion similar to that shown in FIG. 4b. The resulting field lines (andfiber orientations) are shown as dashed lines, and are radial from therod electrode 82 outward to the plate electrodes 80a-d.

FIG. 5b is similar in using a single rod electrode 84, but has only asingle plate electrode 86. The resulting field lines (and fiberorientations) are again shown as dashed lines, but in this example areno longer uniformly radial, rather they all tend to terminate at thesingle plate electrode 86 and are in greater density in the areaimmediately between the rod electrode 84 and the plate electrode 86.This field distribution pattern simulates the stress field created in abolt hole loaded in tension.

By placing electrodes at load bearing surfaces of the final part,another advantage of the present invention can be realized, that is theability to automatically sort prepreg rods. Strong reinforcing fiberstend to be expensive, and ideally would be placed only where theirsuperior strength characteristics were needed. This can be accomplishedto some degree of effectiveness by providing a mixture of strong(expensive) and weak (inexpensive) prepreg rods. The strong rods aremade conductive by one of the methods described above, and the weak rodsare made nonconductive. The strong rods will thus be influenced by theelectric field while the weak rods will not. When electrodes are placedat load bearing surfaces such as with the plate and rod electrodes inFIG. 5b, the electric field is stronger at the high stress locations inthe part, such as between the electrodes as shown by the dashed lines inFIG. 5b. Not only will the strong rods tend to align with the fieldlines, they will also migrate toward areas of increased field strength ,displacing the weak rods to areas where reinforcing is less critical.Thus, placement of electrodes at load bearing surfaces can automaticallyoptimize not only the orientation, but also the placement of fibers inthe composite part. In a like fashion, the mixture could be of stiff andless stiff rods, so as to preferentially locate stiff rods in a desiredstiff portion of the final composite part.

Those skilled in the art will recognize that the placement, type, andshape of electrodes may be varied, depending upon the function of thecomposite part desired, without departing from the scope of the presentinvention. Specifically, any shape of electrode can be used, includingplates, rods, spheres, and irregular surfaces, as determined by theshape of the load bearing surface of the composite part.

One problem unique to highly conductive fiber alignment is the potentialfor shorting. As the fibers align in the electric field, they come intophysical contact with each other forming chains. These chains locallyshort out the electric field resulting in distortions, and if the chainsbecome long enough, they could short out (or arc out) the entireelectric field. Even if voltage gradient electrodes are used, a shortcan cause major disruption to the field pattern.

Referring now to FIG. 6a, a resistive plate electrode 90 is shown withconductive strips on the top and bottom edges 91a, 91b, with the bottomstrip 91b connected to ground 92 and the top strip 91a to a source ofhigh voltage 94. The horizontal lines are lines of equipotential, andare labeled to show a typical voltage gradient appearing on theelectrode 90. They show a decreasing electric field intensity as youapproach the bottom of the electrode. FIG. 6b shows how theequipotential lines are distorted when a chain of fibers contacts theelectrode at a short point 96. Little if any electric field remains inthe region below the short point 98, resulting in less effectivealignment of the fibers. Assuming the alignment system is optimized sothat the fibers tend to reach stable alignment by the time they reachthe bottom, the presence of the short substantially reduces the amountof time a fiber is exposed to the electric field, which results in anincrease in misaligned fibers. In addition, these distortedequipotential lines will result in distortions in the electric fieldorientation which may further impair proper fiber alignment.

One solution is to use a plurality of rod electrodes to simulate theeffect of a plate electrode, as is shown in FIG. 7. A plate electrode iscomprised of multiple rod electrodes 102a-g, in a planar arrangement andspaced apart so as not to be in contact with each other. Rod electrodes102a-g are voltage gradient electrodes, constructed from a continuousresistive material, or in a manner similar to the voltage gradientelectrodes of FIGS. 4a and 4b, or each may simply be a string ofresistors soldered together end to end. If a chain of fibers causes ashort to occur, only one of the rod electrodes 102a-g will show adistorted voltage gradient, and thus the bulk of the electric fieldremains intact.

One skilled in the art will recognize that many arrangments are possiblewithin the scope of the present invention, including varying the numberof rod electrodes, the type and manner of rod electrode construction,and the spacing between rod electrodes.

According to one embodiment of the present invention, reinforcing fibersare preimpregnated with matrix material and formed into rods calledprepreg rods. The rods may be formed by pultruding continuous fiberfilaments through a molten matrix material, in a manner as is known inthe art, and then cut into segments to form the rods. The prepreg rodsare then aligned in a manner previously described. When aligned, thealignment fluid is drained, and the accumulated mat of prepreg rods ismolded into the desired part. Alternately, rather than draining thealignment fluid, the bottom of the tank could be equiped with a conveyorso as to remove the accumulated mat as it forms, resulting in acontinuous operation.

Referring now to FIG. 8a, a top view of an alignment tank with multiplerod electrodes 102a-n arranged in a square grid fashion. By individuallycontrolling the potential of each electrode 102a-n, it is possible toproduce either vertical or horizontal field orientation, and to changeorientation at will during the production process. For example, avertical orientation would be achieved by connecting the firsthorizontal row of electrodes to high voltage, and the next horizontalrow of electrodes to ground, and repeating this pattern for theremaining electrodes. Likewise, a horizontal orientation would beachieved by connecting the first vertical column of electrodes to highvoltage, and the next vertical column of electrodes to ground, andrepeating this pattern for the remaining electrodes. Those skilled inthe art will recognize that by proper control of the electrodes, ±45°diagonal orientations can be easily achieved.

FIG. 8b shows a grid of rod electrodes similar to that in FIG. 8a,except in a hexogonal arrangement, allowing for ±30° and ±60° diagonalorientations. By chosing other grid patterns or geometries, other anglescan be obtained as desired.

FIG. 8c shows a circular grid pattern capable of providing both circularand radial alignment orientations. A first group of electrodes 104 isconected to one polarity of a high voltage source, and a second group ofelectrodes 106 is connected to the opposite polarity. This creates afield in the sector defined between the two groups of electrodes 104,106, which causes fibers within this sector to align perpendicular tothe radial direction. After a certain time, the sector defined by theenergized electrodes is rotated to a new pair of electrode groupsadjacent the first pair. In this example, electrode group 106 would begiven the polarity that group 104 had previously, and group 108 would begive the polarity that group 106 had previously. This rotation wouldthen continue until processing at this alignment pattern is complete.

FIG. 8d shows the same grid pattern of rod electrodes as in FIG. 8c. Inthis example, the electrodes are controlled so as to produce a radialalignment pattern. Electrode 110 is connected to one polarity of a highvoltage source, and a group of electrodes 112 is connected to theopposite polarity. This defines an annular ring within which fiber willtend to align radially. After a certain time, the high voltage isconnected so as to define a larger annular ring, whereby the aligningzone is moved outward much like the aligning sector was rotated in FIG.8c.

With electronic control over the electrodes, those skilled in the artwill recognize that virtually any alignment pattern can be created, andthat several different alignment patterns can be used in the productionof a single composite part.

In order to provide a predictably effective system of aligning prepregrods, the electrical and mechanical properties of the prepreg rods mustbe well known and consistent. The shape of the prepreg rod is important,especially the aspect ratio. Referring now to FIG. 9, a prepreg rod 120is shown with a plurality of reinforcing fibers 122 contained within.The fibers run axially in the rod along its longest dimension, and arepreferably continuous from one end to the other. The amount of fiber 122in the prepreg rod as compared to the total rod volume is important inadding strength to the final part. Ideally, 50-70% of the prepreg rod byvolume would be fiber, although significant benefits can be seen at aslow as 10% fiber by volume. For processes where the prepreg rods consistof sized fiber rods which are aligned and then fully impregnated later,the sized fiber rods should ideally contain 1-10% matrix material byvolume. In order for the prepreg rod 120 to fall through the alignmentfluid without spiraling, the cross sectional shape of the prepreg rodmust be uniform. Prepreg cross sectional shapes will be discussed inmore detail in reference to FIGS. 10a-c. The aspect ratio of the prepregrod is defined as the ratio of the total length 124 divided by thesmallest dimension of the cross section 126. The aspect ratio must begreater than 100 for the final composite part to have mechanicalproperties approaching those of continuous fiber composites, althoughprepreg rods having any aspect ratio greater than one are capable ofbeing aligned.

It is also important to control the cross sectional shape of the prepregrods. Referring now to FIGS. 10a-d, various possible cross sectionalshapes are shown. If the prepreg rods are formed by pultrusion asdescribed above, then they can be pulled through a die so as to createprepreg rods of any desired shape. FIG. 10a shows a circular crosssection. Such prepreg rods will work in an alignment system according tothe present invention, but are not ideal since the processing ratedepends upon the total volume of the prepreg rod and how fast it sinks.Preferred shapes include an ellipse as shown in FIG. 10b. An ellipticalcross sectioned prepreg rod will fall through the alignment fluid in amanner so as to present the lowest drag (and thus the minimum area) tothe flow. For elliptical cross sections with a cross sectional aspectratio of less than approximately five, the elliptical rod can sink asfast as a circular rod, but will carry up to five times the volume tothe mat. A tear-drop shape as shown in FIG. 10c can achieve even furtherincreases in production effciency. A rectangular cross section as shownin FIG. 10d is not ideal a flow viewpoint, but may desireable due toease of manufacture of rectangular cross section prepreg rods.

Electric fields can also be used to produce composite parts with threedimensional fiber alignment patterns, by using an apparatus similar tothat shown in FIG. 1. The tank 12 is made into the same shape as thecomposite part, only larger in every dimension, so as to allow morefreedom of movement of prepreg rods within the tank. A volume of prepregrods 24 sufficient to produce the part is evenly mixed into the tankalong with an alignment fluid 20. According to this aspect of thepresent invention, the prepreg rods are required to remain suspendedwithin the alignment fluid, rather than sink toward the bottom. Thus,the alignment fluid 20 preferably has a density equal to that of theprepreg rods 24.

Electrodes 14a, 14b are located within the tank at locations andorientations corresponding to the load bearing surfaces of the finalpart. The shape of the electrodes is determined by the shape of theparticular load bearing surface. An electric field is applied betweenthe electrodes, creating a three dimensional alignment pattern withinthe tank, to which the prepreg rods 24 align. Once alignment iscomplete, the prepreg rods 24 are bound onto their aligned positions.The alignment fluid 20 can then be removed by evaporation or hydrostaticpressure, causing the prepreg rod structure to shrink nearly to the sizeof the final part, while retaining the alignment of the fibers. Theresulting block of prepreg rods is then molded according to knownmethods to produce the final part.

Bonding of the prepreg rods 24 into their aligned orientations can beaccomplished in one of several ways. The alignment fluid could bechemically caused to gel, thus preventing the prepreg rods fromdisorienting. Alternatively, the alignment fluid could be an ultravioletcurable plastic, and a laser could be scanned in a grid pattern throughthe alignment fluid, creating a gridwirk of cured plastic which wouldhold the prepreg rods 24 in their aligned orientation. Another methodmight be to the prepreg rods 24 with a sponge-like material, which wouldabsorb the alignment fluid and expand to hold the prepreg rods 24 inplace. Those skilled in the art will recognize that any method whichholds the rods in place without disturbing their alignment will workaccording to this aspect of the present invention.

Although the description of the prefered embodiments have beenpresented, it is contemplated that various changes could be made withoutdeviating from the spirit of the present invention. Accordingly, it isintended that the scope of the present invention be dictated by theappended claims rather than by the description of the preferedembodiment.

What is claimed is:
 1. In a tank with a bottom containing a dielectricfluid, a method of aligning prepreg rods into a desire orientation forthe manufacture of composite materials, comprising the steps of:a)applying an electric field of vertically decreasing intensity, from thetop of said tank toward the bottom wherein the electric field followsflow lines corresponding to the desired orientation of prepreg rods; b)dispersing prepreg rods having a density greater than that of thedielectric fluid onto the dielectric fluid; and c) collecting a mat ofaligned prepreg rods as they fall to the bottom of the tank.
 2. In atank with a bottom containing a dielectric fluid, a method ofmanufacturing a composite material part, wherein prepreg rods arealigned into an orientation corresponding to stress lines of thecomposite material part the method comprising the steps of:a) placing anelectrode at at least one location corresponding to load bearingsurfaces of a desired composite material part; b) applying an electricfield between the electrodes, wherein the electric field follows flowlines corresponding to the stress lines; c) dispersing prepreg rodshaving a density greater than that of the dielectric fluid onto thedielectric fluid; and d) collecting a mat of aligned prepreg rods asthey fall to the bottom of the tank and forming the same into acomposite material part.
 3. A method according to claim 2, wherein theapplying step applies an electric field of vertically decreasingintensity.
 4. In a tank the same shape as but proportionally larger thana desired composite material part, a method of aligning prepreg rodshaving a density into a desired three dimensional orientation for themanufacture of a composite material part, comprising the steps of:a)filling the tank with a dielectric fluid having a fluid density equal tothat of the prepreg rods; b) mixing into the dielectric fluid a quantityof prepreg rods sufficient to produce the composite material part; c)applying an electric field, thereby aligning the prepreg rods into adesired three dimensional orientation; d) binding the prepreg rods intotheir aligned orientation; and e) removing the alignment fluid.
 5. In atank with a bottom containing an alignment fluid, a method of forming acomposite material part whereby prepreg rods with desireable propertiesare placed in a critical location of the part, and prepreg rods withless desireable properties are placed in a less critical location of thepart, the method comprising the steps of:a) placing an electrode at atleast one location corresponding to load bearing surfaces of a desiredcomposite material part; b) applying an electric field betweenelectrodes, wherein the electric field follows flow lines correspondingto stress lines, and wherein field intensity is greater in the criticallocation of the part than in the less critical location of the part, c)dispersing onto the alignment fluid a mixture of prepreg rods having adensity less than that of the alignment fluid, including conductiveprepreg rods with desireable properties, and substantiallynon-conductive prepreg rods with less desireable properties, whereby theprepreg rods with desireable properties align according to the electricfield flow lines, and migrate toward locations of greater electric fieldintensity; and d) forming the aligned prepreg rods into a compositematerial part.
 6. The method of claim 1 including applying an electricfield of vertically decreasing intensity by use of gradient electrodes.7. The method of claim 2 including placing a rod electrode at said onelocation.
 8. The method of claim 2 wherein said electric field is ofvertically decreasing intensity.
 9. The method of claim 5 includingplacing a rod electrode at said one location.
 10. The method of claim 5wherein said electric field is of vertically decreasing intensity.